http2: work around Go 1.8 tls.Config.Clone bug

Updates golang/go#19264

Change-Id: Ib5b483d2d830d7a51d59eb7bc5eac106da5d5476
Reviewed-on: https://go-review.googlesource.com/37944
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
2 files changed